Screen: Allow disabling of filter-caps during GStreamer video capture.

By default we set the colorcaps used on our appsink also as filter-caps
for the video source during videocapture. This has the purpose of
trying to force the video source to already output data in a format
suitable for consumption by us, thereby avoiding costly colorspace and
image format conversions.

With some cameras this can cause "negotiation failure" and thereby
failure to open the capture device. A new optionsl 'recordingflags'
value 4096 (2^12) allows to avoid setting filter-caps or their
camerabin2 equivalent, so that problematic capture devices can operate
at their preferred output image format and a colorspace conversion
takes place to convert to our required format. More inefficient, but at
least should work.

-> Screen for OSX and Windows rebuilt and tested a bit on OSX with
camerabin2.
-> Real test, e.g., with VRMagic cams under Windows by Marieke pending.